The present invention relates to a flat panel display unit such as a liquid crystal display panel, and to a method for repairing line patterns formed in the flat panel display unit.
Liquid crystal display (LCD) panels are used in personal computers and other types of office automation equipment, as well as in televisions and other audio/video (A/V) equipment. An LCD panel has pixel electrodes for driving the liquid crystals with an electric field; thin film transistors (TFT), whose number corresponds to the pixel count, for applying a voltage independently to each pixel electrode; a TFT panel on which orthogonal scan lines and signal lines are formed on a glass panel for controlling TFT switching; and a back panel comprising a color filter and back plane electrode opposing the TFT panel, with a specific gap therebetween in which the liquid crystals are filler.
As flat panel displays have become larger and display resolution has improved, the pixels have become smaller. This makes it increasingly difficult to manufacture defect-free products, and there is a need to increase manufacturing yield. A common conventional way of improving yield has been to repair panels determined to be defective.
Japanese Patent Laid-open Publication (kokai) H4-72552 describes a method for repairing line defects in a TFT panel manufacturing process where the circuit pattern is formed with the scan lines bifurcating the intersections of the scan lines and signal lines. When, during inspection, a short is detected at a scan-line-to-signal-line intersection prior to injecting the liquid crystals, a voltage is applied to each scan line and signal line and current flows to any short between the scan line and signal line. The defect can then be located by infrared inspection that looks for the heat produced by current at the short. The short is then electrically isolated by cutting that scan line with a YAG laser, for example, before and after the intersection with the signal line, leaving the other bifurcated scan lines uncut.
Japanese Patent Laid-open Publication (kokai) 184842/1996 describes a method of repairing a break in the wiring by coating the break with an organic solution dispensed with a glass pipette, then exposing the coated area to a laser to thermally break down the organic complex solution, deposit a metallic film, and thus form an electrical connection across the break.
With the conventional method of repairing shorting defects, the scan lines of the TFT panel bifurcate where the scan lines and signal lines intersect, the scan lines and signal lines having an interlayer insulation film therebetween, so that shorting defects can be repaired, and a short on either one of the legs can be repaired. As mentioned above, the intersecting scan line is cut with a YAG laser, for example, before and after the short at the intersection between a signal line and one of the two scan line legs, thereby electrically separating the shorting defect from the TFT circuit. Wiring material melted by the laser can, however, pass through the insulation film where the line is cut and scatter around the cut area. The protective insulation film can thus be lost around the cut, electrically exposing the scan line to the liquid crystal. As a result, there is insufficient insulation protection around the repair site, and leakage current can flow from the cut area. The liquid crystals vary with the type of product being produced, and, depending on the type of liquid crystal used, this leakage current can cause point defects in the liquid crystals, orientation defects in LCD alignment, and other problems.
Apart from the shorting defects, foreign objects adhering to the panel during the process forming the lines on the TFT panel can prevent normal formation of the insulation film that protects the lines. This results in defects that permit leakage current to flow from the insulation film defect to the liquid crystal. Depending upon the type of liquid crystal, this leakage current, too, can cause point defects in the liquid crystals, orientation defects, and other problems.